Mixing 0W40 And 5W30 – Understanding Oil Viscosity: What Do 0W-40

ByRobert Lozano

In an emergency, mixing 0W-40 and 5W-30 synthetic oils to top off a low engine is generally safe and far better than running on low oil. However, this should only be a temporary solution, as it alters the oil’s intended viscosity and performance characteristics. An oil change with the manufacturer-recommended grade should be performed as soon as possible.

You’re on a long road trip, miles from the nearest town, when your “Check Oil” light flashes. You pull over, pop the hood, and the dipstick confirms it—you’re a quart low. The only oil you can find at a dusty gas station is 0W-40, but your engine calls for 5W-30. What do you do?

This exact scenario causes a lot of stress for car owners. You’ve heard horror stories about engine damage, but you also know that running on low oil is a recipe for disaster. This is where practical knowledge separates a panicked driver from a prepared one.

This comprehensive guide will walk you through everything you need to know about mixing 0W-40 and 5W-30. We’ll break down what the numbers mean, explain the risks and benefits, and give you clear, actionable advice so you can make the right call for your engine’s health, whether you’re on the highway or a remote trail.

Understanding Oil Viscosity: What Do 0W-40 and 5W-30 Really Mean?

Before we dive into mixing, let’s quickly demystify those numbers on the bottle. Motor oil grades, set by the Society of Automotive Engineers (SAE), tell you about the oil’s viscosity—its resistance to flow—at different temperatures. This is the foundation of our entire discussion.

The “W” is for Winter

The first number followed by a “W” (e.g., 0W or 5W) indicates the oil’s viscosity at cold temperatures. A lower number means the oil is thinner and flows more easily when the engine is cold.

  • 0W-40: This oil is extremely fluid at startup, even in freezing conditions. It pumps quickly through the engine to protect vital components the moment you turn the key.
  • 5W-30: This oil is also excellent for cold starts but is slightly thicker than a 0W oil at the same low temperature. It’s a very common grade for modern passenger cars.

The Operating Temperature Number

The second number (e.g., 40 or 30) represents the oil’s viscosity at the engine’s normal operating temperature (around 212°F or 100°C). A higher number means the oil is thicker and provides a more robust protective film under high heat and stress.

  • 0W-40: At operating temperature, this oil behaves like a 40-weight oil, offering a thicker, more protective film, often preferred for high-performance or European vehicles.
  • 5W-30: This oil acts like a 30-weight oil when hot. It provides excellent protection while being thin enough to maximize fuel efficiency in many modern engines.

Is Mixing 0W-40 and 5W-30 Safe for Your Engine? The Short and Long Answer

Here’s the bottom line: Yes, in a pinch, it is safe to mix 0W-40 and 5W-30 to top off your engine. It is infinitely better to have a mix of compatible oils at the correct level than to run your engine on low oil.

However, this comes with important caveats. This is a practice for emergencies, not a strategy for your regular oil change. This section provides a complete mixing 0W-40 and 5W-30 guide to help you understand the nuances.

When It’s Okay to Mix

The ideal scenario for mixing is when you need to add a quart or less to get your oil level back into the safe zone on the dipstick. Both 0W-40 and 5W-30 are typically full synthetic oils and are designed to be miscible (meaning they will mix together without separating).

Ensure both oils meet the same or a compatible API (American Petroleum Institute) service rating, such as API SP or SN. You can find this “donut” symbol on the back of the bottle. Mixing oils with the same API rating ensures their additive packages are generally compatible.

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When You Should Avoid It

You should not intentionally mix these oils for a full oil change. Your vehicle’s engineers specified a particular viscosity for a reason—it provides the optimal balance of protection, performance, and fuel economy for your specific engine design.

Intentionally creating a “custom blend” is a bad idea. You’ll end up with an oil of an unknown viscosity, somewhere between the two original grades. This can negatively affect engine tolerances, VVT (Variable Valve Timing) systems, and overall performance.

Potential Benefits and Common Problems with Mixing 0W-40 and 5W-30

While the main benefit is preventing engine damage from low oil, there are other factors to consider. Let’s look at the upsides and the potential common problems with mixing 0W-40 and 5W-30.

The Upside: The Benefits of Mixing 0W-40 and 5W-30 in an Emergency

  • Immediate Engine Protection: The number one benefit is avoiding catastrophic failure. Running an engine low on oil can cause metal-on-metal contact, leading to scored bearings, damaged camshafts, and complete seizure.
  • Getting Home Safely: For an off-roader stuck on a trail or a family on vacation, having the ability to top off and continue your journey is invaluable.
  • Slight Viscosity Tweak: If you add 0W-40 to a 5W-30 system, you’ll slightly increase the oil’s viscosity at operating temperature. This could theoretically offer a marginal increase in protection in very hot climates or under heavy load, but it’s not a reliable performance hack.

The Downside: Potential Risks to Your Engine

  • Altered Viscosity: The resulting mixture will not be a true 5W-30 or 0W-40. This can impact how well the oil protects parts designed with specific clearances in mind.
  • Diluted Additive Package: Motor oils contain a carefully balanced cocktail of additives—detergents, dispersants, anti-wear agents, and more. Mixing oils can dilute this package, reducing its effectiveness over the long term.
  • Impact on Fuel Economy: A thicker oil (like a mix containing 0W-40) creates more drag, which can slightly decrease your fuel efficiency compared to running pure 5W-30.
  • VVT System Issues: Modern engines with Variable Valve Timing rely on oil pressure and viscosity to function correctly. Using the wrong grade can sometimes lead to sluggish performance or even a check engine light.

Your Emergency Mixing 0W-40 and 5W-30 Guide: Best Practices for Topping Off

If you find yourself in a situation where you need to mix, follow these mixing 0W-40 and 5W-30 best practices to do it safely.

  1. Confirm the Need: Park your vehicle on a level surface. Wait a few minutes for the oil to settle, then pull the dipstick, wipe it clean, re-insert it fully, and pull it out again. Check the level. Only add oil if it’s at or below the “ADD” mark.
  2. Check for Compatibility: As mentioned, ensure both oils are the same type (preferably full synthetic) and share a similar API rating (e.g., both are API SP). Never mix a specialty racing oil with a standard passenger car motor oil.
  3. Add a Small Amount: Don’t dump the whole bottle in at once. Start by adding half a quart, then wait a minute and re-check the dipstick. Add more as needed to bring the level to the “FULL” mark. Do not overfill the engine, as this can cause its own set of problems.
  4. Drive Conservatively: After mixing, avoid high-RPM driving, heavy towing, or aggressive acceleration. Your goal is to get to a location where you can perform a proper oil change.
  5. Change the Oil Soon: Treat the mixed oil as a temporary fix. Plan to perform a full oil and filter change with the manufacturer-recommended grade as soon as you can. This is the most crucial step in this mixing 0W-40 and 5W-30 care guide.
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The Impact on Performance: Off-Roaders and Tuners Take Note

For the enthusiast crowd, the temptation to “tune” oil viscosity can be strong. Some may think that adding a thicker 0W-40 to their 5W-30 will provide more protection during a track day or a tough rock-crawling session.

While a slightly thicker oil film can be beneficial under extreme heat, you’re better off switching entirely to a manufacturer-approved oil for those conditions (like a 5W-40 or 0W-40) rather than creating an unpredictable homebrew. Your engine’s oiling system, including the oil pump and passages, is designed for a specific viscosity range. Guesswork is not your friend here.

Sustainable and Eco-Friendly Mixing 0W-40 and 5W-30 Practices

When it comes to being green, the best practice is to avoid waste. The most eco-friendly mixing 0W-40 and 5W-30 approach is simply using the mixed oil as a short-term bridge to your next scheduled oil change.

Do not drain and discard the mixed oil immediately. Drive on it until you can conveniently perform a full service. When you do, be sure to recycle the used oil. Most auto parts stores, quick lube shops, and municipal recycling centers accept used motor oil for free. This prevents harmful chemicals from contaminating soil and groundwater, making it a sustainable choice.

Frequently Asked Questions About Mixing 0W-40 and 5W-30

What happens if I put 0W-40 in a 5W-30 engine?

If you top off a 5W-30 engine with a quart of 0W-40, very little will happen in the short term. The engine will be slightly better protected at high temperatures and will have slightly better flow on a cold start. However, you should change it back to the recommended 5W-30 at your earliest convenience to ensure optimal performance and efficiency.

Can mixing different oil weights cause engine sludge?

Mixing two high-quality, modern synthetic oils of different weights is highly unlikely to cause sludge. Sludge is typically caused by oil breakdown from excessive heat, contamination (with coolant or fuel), or extending oil change intervals far beyond their limit. As long as both oils are API-certified, they are formulated to be compatible.

Is it better to mix 0W-40 and 5W-30 or run on low oil?

It is always better to mix the oils and have the proper oil level. Running an engine on low oil is one of the fastest ways to cause severe, expensive, and often irreversible damage. The risks of mixing are minimal in the short term, while the risks of low oil are catastrophic.

The key takeaway is to view mixing oil viscosities as a tool for emergencies, not a regular maintenance practice. Your owner’s manual is your best friend—it contains the precise oil specification your engine needs to live a long and healthy life. Use the right oil whenever possible, but don’t hesitate to mix compatible grades to save your engine when you’re in a bind.

Stay safe, check your oil regularly, and keep wrenching!

Robert Lozano
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